In recent years, gallium nitride (GaN)-based compound semiconductor light-emitting devices has attracted attention as short wavelength light-emitting devices. This gallium nitride based-compound semiconductor is formed using a method such as the metal-organic chemical vapor deposition method (MOCVD method) or the molecular beam epitaxial method (MBE method) on substrates of various oxides or group III to group V compounds starting with a sapphire single crystal.
A sapphire single crystal substrate has a lattice constant which differs from the lattice constant of GaN by 10% or more. However, since a nitride semiconductor having excellent properties can be formed by forming on a sapphire single crystal substrate, a buffer layer comprising AlN or AlGaN, a sapphire single crystal substrate is widely used. For example, when a sapphire single crystal substrate is used, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer are formed on the sapphire single crystal substrate in this order. Since a sapphire single crystal substrate is an insulant, in general, in a device comprising a sapphire single crystal substrate, both a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on an n-type semiconductor layer are positioned on one side of the device. Examples of a method for extracting light from a device comprising positive and negative electrodes on one side includes a face-up method in which light is extracted from the p-type semiconductor side using a transparent electrode such as ITO as a positive electrode, and a flip-chip method in which light is extracted from the sapphire substrate side using a highly reflective film such as Ag as a positive electrode.
As is explained above, sapphire single crystal substrates are widely used. However, since sapphire is an insulant, a sapphire single crystal substrate has some problems.
First of all, in order to form the negative electrode, the n-type semiconductor is exposed by etching the light-emitting layer; therefore, the area of light-emitting layer is reduced by the area of the negative electrode, and output power decreases.
Secondly, since the positive electrode and the negative electrode are positioned on the same side, electrical current flows horizontally, current density is increased locally, and the device generates heat.
Thirdly, since heat conductivity of a sapphire substrate is low, generated heat is not diffused, and the temperature of the device increases.
In order to solve these problems, a method is used in which a conductive substrate is attached to a device comprising an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer which are stacked on a sapphire single crystal substrate in this order, the sapphire single crystal substrate is removed, and then a positive electrode and a negative electrode are positioned on both sides of the resulting stacked layers (For example, see Patent document 1).
In addition, the conductive substrate is formed by plating, not by attaching (For example, see Patent document 2).
Examples of a method for attaching a conductive substrate include a method in which metal compounds having a low melting point such as AuSn are used as an adhesive, or an activation junction method in which a surface to be joined is activated by argon plasma under vacuum. These methods require that the surface to be attached be extremely flat and smooth. Therefore, if there is foreign matter such as particles on the surface to be attached, the area is not closely attached. Due to this, it is difficult to obtain a uniformly attached surface.
When a substrate is made by plating, it is advantageous that there is little influence from extraneous material; however, light-extracting efficiency decreases because the side of a p-type semiconductor is covered by plating.
In order to improve the light-extracting efficiency, the general method is to form a Ag layer having high reflectivity on the ohmic contact layer before plating processing. However, light absorption in the light-emitting layer becomes a problem because most reflected light must transmit through the light-emitting layer.
In order to prevent reflected light be generated as much as possible, a semiconductor device using a transparent substrate as a supporting substrate is proposed (For example, Patent see document 3).
However, when a transparent substrate is used for a supporting substrate, for example, when using SOG (a spin-on glass), there is a problem in that a substrate having sufficient strength cannot be formed because about 5 μm is the thickness limit of a thick film.
Patent document 1, Japanese Patent (Granted) Publication No 3511970.
Patent document 2, Japanese Unexamined Patent Application, First Publication No. 2004-47704.
Patent document 3, Japanese Unexamined Patent Application, First Publication No. 2003-309286.